A magnetic resonance imaging apparatus (hereinafter, MRI apparatus) is an apparatus that nondestructively visualizes an atom distribution inside a subject body, using a property of atoms such as hydrogen positioned in a magnetic field of selectively absorbing and radiating only electromagnetic waves of a frequency that is dependent on a type of an atom and a magnetic field among electromagnetic waves of various frequencies. A signal acquired by measuring electromagnetic waves that are radiated from the subject body by coils to be digitalized is called k-space data.
The k-space data is two-dimensional or three-dimensional data that can be acquired, for example, by repeating one-dimensional imaging. An atom distribution image inside a subject body can be acquired by performing the Fourier transform (hereinafter, the Fourier transform includes the inverse Fourier transform) on the k-space data. The acquired atom distribution image is called an MR (magnetic resonance) image, and the process of calculating the MR image from the k-space data is called reconstruction, image reconstruction, image generation, or the like. The central portion of the k-space data corresponds to low frequency components at the time of performing the Fourier transform on the MR image, and the peripheral portion of the k-space data corresponds to high frequency components at the time of performing the Fourier transform on the MR image.
In an MRI apparatus, k-space data that is required for reconstruction is acquired by repeatedly performing one-dimensional imaging, and it is known that this imaging generally requires time. Moreover, it is known that if a state of a subject body changes with time, the quality of a reconstructed MR image deteriorates. Therefore, for imaging time-series data having large data amount when a state of a subject body changes, for example, imaging of a heart beating, there is a strong demand for shortening time required therefor. To perform higher-speed imaging, for example, a technique of parallel imaging has been studied and developed that k-space data is acquired by down-sampling imaging with multiple coils at the same time using sensitivity that varies depending on arrangement of coils, and an MR image is reconstructed from the multiple pieces of acquired k-space data while suppressing artifacts.
As a technique of parallel imaging for time-series k-space data, a technique called k-t BLAST (k-space time broad-use linear acquisition speed-up technique) or k-t SENSE (sensitivity encoding) has been known. It is called k-t BLAST when the number of coils is few with respect to a rate of down-sampling samples, and k-t SENSE when it is not; however, in the explanation below, it is called k-t SENSE including k-t BLAST unless otherwise explicitly distinguished. Although a case of multiple coils is mainly explained hereafter, a case of a single coil is also acceptable as a special case of k-t BLAST. Note that a case of a single coil is also called k-t SENSE for convenience sake.